1. Field of the Invention
This invention relates to a multi CPU system in which a plurality of CPUs are connected with the same system bus.
2. Description of the Related Art
Conventionally, in the multi CPU system in which a plurality of CPUs are connected with the same system bus, there is used, in order to perform exclusive control of each CPU accessing to the same resource on a system bus, a method for performing the control using an OS (Operating System) or a method for causing each CPU to check a semaphore flag prepared in a memory which is commonly accessible from each CPU before performing a bus access (for example, see Japanese Laid-Open Patent publication (Kokai) No. H05-020279).
The conventional method performed by using an OS, however, depends on the OS per se, which burdens softwares. In the method of preparing a semaphore flag in a memory which is commonly accessible to each CPU, there are caused problems that a time lag occurs when each CPU accesses to the memory, and that, in a case where the respective CPUs access to the semaphore flag simultaneously, the exclusive control cannot be performed.